Differences between surge and lightning arresters

In power system, surges are transients that occur all of a sudden. These transients can reach amplitudes of very high voltages and will last only for a short period of time. Within that short period, it can damage or degrade electrical or electronic equipment. 

Most of the electrical devices are designed in such a way that they can withstand very slight variations in their nominal operating voltage. But they will get damaged when they are subjected to the surge voltages that are oscillating and decaying over time in nature.

Fig 1. Voltage surge

Surge voltages have internal as well as external sources. About 70% of the surges are generated within a facility itself. Frequent increase and decrease of the flow of power can cause surges. Power system recovery from an outage, loose connections in the live circuits, frequent starting and stopping of loads, etc can lead to surge voltages. All these are the internal sources of surges.

Lightning strikes are treated as the major external source of the surge voltage. Apart from that, the disconnection and reconnection of utility power to the consumers also can cause voltage surges. Sometimes utilities tend to switch the supply of power from one grid to another. This switching cause voltage transients and spikes.

SURGE ARRESTER

Fig 2. Surge arrester

Surge arresters are protective devices used for limiting surge voltages on electrical devices. Frequent rise and fall of the flow of power cause voltage surges. Use of the surge arrester ensures that the transient voltages do not get into the power system. Once the transient gets passed through the surge arrester, it clamps out the unwanted voltages above a safe threshold value. It basically diverts the high voltage to ground.


LIGHTNING ARRESTER
Lightning strikes reach a voltage level of thousands of kilovolts which can damage all the devices. Lightning arresters protect the electrical devices from the harmful effects of lightning strikes.  It redirects the lightning voltage to ground.

Fig 3. Lightning arrester connected at the top of a building

It has got two terminals; a high voltage terminal and a low voltage terminal. The low voltage terminal is connected to a wire of low resistance that is buried in the ground. When a lightning surge strikes the lightning arrester, the current from the surge travels through the high voltage terminal to ground via the low voltage terminal. Lightning arresters are connected at the top of buildings so that the lightning surge first strikes the arrester before it could reach to the circuits inside the building.

Differences between lightning arresters and surge arresters

• A lightning arrester is connected outside the utility panel. But the surge arresters are connected inside the panels. 
• Lightning arrester protects the system from lightning strikes. But surge arrester can protect from electrical surges as well as switched over voltages.